general rule for when to perform an operation in either Mysql or php

I am playing around with Mysql and php and I am not sure about when it is best do something in mysql or php when BOTH can perform the operation. Example, date formating can be performed by Mysql during the query DATE_FORMAT(blah, blah...) or by php with date(blah, blah..). What are the upsides/downsides of doing something like this in Mysql/php. My current (though limited) thinking is not much improvement in either approach over the other in both operations are performed by the same server. I can only see a difference when using huge databases and the database is on a different machine. What is considered to be "Best practise" when Mysql and php can perform the same operation?